Conventionally, writing implements, particularly ballpoint pens that are formed so that a writing tip is protruded and retracted by push-out operation of a knock section provided at a rear end have been widely used. Moreover, mechanical pencils with lead advanced from a front end by push-out operation of a similar knock section have been also widely used.
In such a retractable writing implement, an elastic member such as a spring is used as a component in a knock mechanism. Then, for a ballpoint pen, for example, when a knock section is pressed in a direction against a biasing force of the elastic member, a push-out section connected to the knock section pushes out a writing tip, which is in turn protruded from a front end for a writable state. At this time, with the elastic member compressed, the push-out section usually remains in a push-out position by a lock mechanism and is disconnected from the knock section. Thus, the knock section is free in the writable state, and is moved toward a front end by gravity when the writing tip points downward and moved toward a rear end when the writing tip points upward.
Then, when the knock section is again pressed, the lock mechanism is released, the writing tip is retracted by return of the elastic member by its biasing force, and the push-out section presses the knock section rearward and returns the knock section to its original position.
For such a knock mechanism, after the knock section is pressed to return the writing tip to its original position, the knock mechanism sometimes vigorously hits a rear end or the like of a shaft tube containing the knock mechanism by a return force of the elastic member. If this repeatedly occurs, damage may be caused by elastic fatigue, or an impact may propagate to a refill to cause ill effects such as thin drawing lines for a ballpoint pen. Moreover, with a strong return force of the elastic member, vibration may propagate to a user's hand to provide uncomfortable feeling to the user.
The invention described in Patent Document 1 in which an air-damper mechanism by air resistance is applied in a part of a knock mechanism is a conventional technique to solve such problems.
Further, the invention described in Patent Document 2 in which a sealed space with respect to an ink-containing tube is obtained at the time of knocking and in which a mechanism that pressurizes the space is provided is also disclosed though its purpose is different.    Patent Document 1: JP 2005-131942 A    Patent Document 2: JP 2000-335173 A